High Pressure and Road to Room Temperature Superconductivity

TL;DR

This paper explores how high pressure induces structural transitions in sulfur hydrides, leading to record high-temperature superconductivity through complex phonon interactions, and proposes a generalized analytical model for Tc.

Contribution

It introduces a new analytical approach to understanding Tc in high-pressure superconductors, accounting for complex phonon spectra and optical mode contributions.

Findings

Superconductivity in sulfur hydrides reaches 203 K under high pressure.

The pressure-induced phase transition sharply increases Tc from 120K to 200K.

Presence of small Fermi pockets suggests a two-gap superconducting spectrum.

Abstract

High pressure serves as a path finding tool towards novel structures, including those with very high Tc.The superconductivity in sulfur hydrides with record value (203 K) is caused by the phonon mechanism. However, the picture differs from the conventional one in important ways. The phonon spectrum in sulfur hydride is both broad and has a complex structure. High value of Tc is mainly due to strong coupling to the high-frequency optical modes, although the acoustic phonons also make a noticeable contribution. New approach is described;it generalizes the standard treatment of the phonon mechanism and makes it possible to obtain an analytical expression for Tc . It turns out that, unlike in the conventional case, the value of the isotope coefficient varies with the pressure and reflects the impact of the optical modes. The phase diagram, that is the pressure dependence of Tc, is rather peculiar. A crucial feature is that increasing pressure results in a series of structural transitions, including the one, which yields the superconducting phase with the record Tc.In a narrow region near 150GPa the critical temperature rises sharply from 120K to 200K. The sharp structural transition, which produces the high Tc phase, is a first-order phase transition caused by interaction between the order parameter and lattice deformations.Remarkable feature of the electronic spectrum in the high Tc phase is the appearance of small pockets at the Fermi level. Their presence leads to a two-gap spectrum, which can, in principle, be observed, with the future use of tunneling spectroscopy. This feature leads to non-monotonic and strongly asymmetric pressure dependence of Tc. Other hydrides can be expected to display even higher values of Tc, up to room temperature. The fundamental challenge lays in creation a structure capable of displaying high Tc at ambient pressure.